Filamentary electrode structure for electron discharge devices



Dec. 13, 1960 G. E. CARTER, JR 2,964,668 FILAMENTARY ELECTRODE STRUCTUREFOR ELECTRON DISCHARGE DEVICES 3 Sheets-Sheet 1 Filed May 17, 1954INVENTOR. GEORG E. CARTE JR.

(9'1 ATTORNEY FIG.

ER, JR E. STRUCTURE FOR 3 Sheets-Sheet 2 R GEORGE E. 0A BY @w 0 ATTORNEY1960 G. E. CART FILAMENTARY ELECTROD ELECTRON DISCHARGE DEVICES FiledMay 17, 1954 2\ l/////f/I \0 3 M 3 mfA qww fill/I A villi/Ill FIG.2

Dec. 13, 1960 ARTER JR 2,964,668

G. E. C FILAMENTARY ELECTRODE STRUCTURE FOR ELECTRON DISCHARGE DEVICESFiled May 17. 1954 3 Sheets-Sheet 3 INVENTOR. GEORGE CARTER JR. BY 0 &

ATTORNEY United tates Patent FILAMENTARY ELECTRODE STRUCTURE FORELECTRON DISCHARGE DEVICES George E. Carter, Jr., Danvers, Mass,assignor to Bomac Laboratories Inc., Beverly, Mass., a corporation ofMassachusetts Filed May 17, 1954, Ser. No. 430,197

4 Claims. (Cl. 313-275) The present invention relates to an electrodestructure for electron discharge devices and more particularly to anon-microphonic filamentary cathode of ruggedized construction forthermionic tubes.

Recent military developments have shown the increasing need forthermionic tubes capable of withstanding acceleration shocks up to100,000 gravities and be able to perform satisfactorily. In prior artdevices, filamentary cathodes are commonly tensioned to prevent bucklingwhen heated. This tensioning results in the filament having a naturalresonant period which renders it unsuitable for use in devices subjectedto conditions of severe accelerations, shocks and vibrations.

A further disadvantage of prior art filaments is the phenomenon referredto in the art as microphonism and noise. Since the electrode spacingsare critical and vital for the performance of the device, any vibrationswill cause variations in the current flow from the filament to otherelements. These variations may be noted in output as viewed on anoscilloscope through a suitable test circuit, and are predominantly theresult of harmonic or .random vibrations of the tensioned filament.

To overcome the disadvantages mentioned, the inven- It is an object ofthe present invention to provide a novel electrode structure inthermionic tubes capable of withstanding extreme conditions of shock,vibration and high accelerations.

A still further object is to provide a novel ruggedized filamentarycathode having rapid warm-up times.

Another object is the provision of a novel filamentary cathode forthermionic tubes that is non-microphonic.

A feature of the invention relates to a filamentary cathode spirally andhelically Wound on a rigid supporting member thus eliminatjng the use ofany tensioning devices. While the prior art discloses filamentarycathodes wound on a refractory mandrel such structure cannot be employedin devices subjected to extreme accelerations, shocks and vibrationsbecause of the inherent brittleness of the mandrel. I have discovered,however, that a sheath or sleeve of a conductive metal may be fittedover a core of a material of low thermal conductivity and a metallicfilament wound over a portion of the sheath will provide an extremelyrigid filament structure.

Further, the metallic sheath provides not only the desired rigidity, butan efiicent means for conducting current to the filament cathode.

The disclosed structure also provides a ruggedized structure having nonatural resonant frequencies.

The objects, features and advantages will be more readily appreciatedafter consideration of the following detailed specification andaccompanying drawings in which:

Figure 1 is an enlarged perspective view of an embodiment of theinvention with a portion of the envelope and components broken away;

Figure 2 is a detailed cross sectional view along the line 22 in Figure1;

Figure 3 is an exploded view showing the relationship of the componentsof the filamentary cathode of my invention; and

Figure 4 is an enlarged perspective view of the assembled cathode of theinvention; and

Figure 5 is a detailed cross sectional view of the embodiment of theinvention.

Referring to Figure 1 and Figure 2, the embodiment of the inventionresides in a thermionic tube 1 constructed generally according to thedisclosure and claims of co-pending application of Richard S. Briggs andGeorge E. Carter, Jr., Serial No. 430,196 filed May 17, 1954. Theenvelope consists of a cylindrical main body member 2 with end members 3and 4 hermetically sealed thereto by brazing or other sealing methods.The assembled envelope defines a central passageway in which theelectrode structure is positioned concentrically around a common axis.Flanged eyelets 5 and 6 are joined coaxially at the ends of members 3and 4 by means of glass to metal seals 7 and 8. Tubular members 9 and 10are joined in a like manner to the flanged eyelets by glass to metalseals 11 and 12 to enclose the ends of the envelope.

The electrode structure of the embodiment comprises a metallic plateinsert 13 of a generally rectangular shape affiixed to the insidesurface of the body member 2. The insert serves as the anode electrodeand external circuit connection means may be attached to this bodymember. A grid electrode 14 is supported at one end by a sleeve 15mounted at the inner end of eyelet 5 with suitable indentations 16 toreceive grid side rods 17. Grid 14 consists of metallic side rods 17with a plurality of turns 13 of a suitable wire helically wound andfixed thereon as shown. The opposite end of the grid is sup ported by anapertured insulating spacer 19 positioned and supported Within envelopeand member 4. The external circuit connections may be made to eyelet 5.

The assembled envelope with plate insert and grid electrode may behydrogen fired to clean the components as well as braze the main bodymember 2 and end members 3 and 4. Such processing prior to the mountingof the cathode electrode eliminates the dangers of any contamination ofthis electrode particularly when emissive coatings containing metallicoxides are employed.

A filamentary cathode embodying the teachings of the invention issupported axially Within the central passageway by tubular members 9 and10. The construction of this electrode will now be described byreferring to Figures 3 and 4.

A pair of tubular metallic sheaths 2t) and 21 of a conductive metal,such as nickel or the alloy Kovar which is composed of nickel, iron andcobalt, are provided with a rolled or peened section 22 intermediate tothe ends thereof. Supported with one end in each sheath is a post 23 ofa low thermal conductivity material such as Alundum with the ends ofsaid post in contiguous relationship with section 22. Other means forpreventing longitudinal movement of the post may be employed. Thesections 22 have been observed to act not only as stop means, but alsoact as expansion joints to still rigidly support post 23 when thecathode is heated.

To prevent heat conduction from the filament to the supporting member, aheat insulating barrier is desirably established. This may beaccomplished in several ways, such as the application of a coating of arough and porous heat insulating material applied to the post at anintermediate portion thereof to form a sleeve abutting 3 the metallicsheaths 20 and 21. An alternative method may be to slidea sleeve of thedesired rough and porous insulating material over the post beforeassembling the rnetallicsheaths over the ends of said post.

I have found that a coating ofa rough and porous refractory material onthe postwill prevent excessive heat conduction from the filament to thepost. In the illustrative embodimenhfl, therefore, provide on theexposed surface of the post 23 when assembled between sheaths 20 and 21,a sleeve 24 of a low conductivity refractory material. Sleeve 24consists of essentially a 97% mixture of alumina and silica with tracesof titanium, calcium, iron and magnesium oxides. This material wasground and powdered to a particle size of 300 mesh or over and was mixedwith a suitable binding agent. The mixture may be applied by spraying,dipping or painting the desired surface. After applying, the coating isallowed to dry to form a sleeve having approximately the same diameteras the abutting sheaths. A thin metallic filament 25 is spirally andhelically vwound on the sheaths 2t) and 21 and coated post 23, so .astoprovide the maximum emitting area over a short length. Severaloverturns of the filament wire are provided on sheaths 20 and 21 withthe ends affixed to said sheaths to provide etficient contact as at 26.Additional points for fixing the filament on the sheaths may be selectedif desired.

,Following the winding and fixing of the filament on the rigid structurean emissive coating 27 may be applied by conventional techniques. Thecoating will be applied to the filament turns as well as the areas ofthe exposed sleeve between said turns. It has been observed thatthecoating in this area will outgas satisfactorily during processing. Inthe illustrative embodiment a filament structure comprising a coatedfilament has been described, however, similar results will be attainedwith the use of a filament suitably impregnated with an electronemissive material.

' The cathode assembly is mounted axially within the central passagewayand is rigidly supported by tubular members 9 and 10. Electricalconnection means for this electrode may be made by means of said members9 and 10. The ends of the tubular members are vacuum sealed as at 28 and29. It is desirable to allow the sealing material to flow in the spacesbetween the cathode sleeve and supporting members to further ruggedizeand fix the cathode assembly.

Degassing of the tube may be accomplished by means of a getter bar 39mounted on tabs 31 and 32 in space 33; between glass to metal seal 8 andinsulating spacer 19. Tabs 31 and 32 may be conductively connected toeyelet 6 and envelope end member 4 respectively, to provide electricalcontinuity for vaporizing of the gettering material.

Exhaust tubulation 34 extends into a passageway provided in end member4. After evacuating the assembled tube and flashing of the getter, thetubulation is tipped and sealed.

Figures 4 and illustrate further the novel features of the filamentarycathode of the invention. While I have provided a passageway 35 throughthe post 23, a solid post will produce satisfactory results. The lowthermal conductivity characteristics of the post 23 are further enhancedby the sleeve of refractory coating 24 which, in turn, supports thefilament turns. The rough and porous surface of the coating in contactwith the filament reduces the contact area and heat conduction.

The pitch of the filament turns tends to prevent slipping andover-riding when the filament is heated and sub jected to severeaccelerat.ons, shocks and vibrations.

Filamentary cathodes constructed according to the teachings of theinvention have reached the full emitting state in less than three tenthsof a second after application of the heating voltage. With the structuredescribed, the filamentary cathode has no natural resonances comparablewith those of conventional stretched wire filamerits and creates novariations in the output of the illustrative tube when employed in anoscillator circuit. Thermionic tubes with the cathode of the inventionhave been adapted with proper circuitry to perform as amplifiers,rectifiers of detectors.

Further, the filamentary cathode of the invention facilitatesminaturizatlon as well as ruggedizing of thermionic tubes in that theelimination of prior art filament suspensionand support structure hasresulted in tubes having an overall length under 1.25 inches. Ease offabrication as well as simpLcity of construction makes it possible fordevices of the character described to be produced with modern high speedproduction techniques.

While an illustrative embodiment of the invention has been disclosedvarious modifications will occur to those skilled in the art. Therefore,it' is my intention to cover in the appended claimssuch modifications orvariations as fallwithin the'spirit and scope of the invention.

What is claimed is:

1. A filamentary electrode structure comprising a core of a low thermalconductivity material, a sleeve of a rough surfaced heat insulatingmaterial mounted upon said core at an intermediate portion thereof, apair of metal sheaths fitted over said core in abutting relation to saidsleeve, a filament wound helically upon said sheaths and sleeve, and alayer of electron emissive material deposited upon said filament and thesurface of said sleeve.

2. A filamentary electrode structure comprising a cylindrical core of alow thermal conductivity material, a sleeve of a rough surfaced heatinsulating material mounted upon said core at an intermediate portionthereof, a pair of tubular metal sheaths fitted over said core inabutting relation to said sleeve, a flat filamentary metal ribbon woundhelically upon said sheaths and extending across said sleeve, and alayer of electron emissive material deposited upon said ribbon and thesurface of said sleeve.

3. A filamentary electrode structure comprising a cylindrical core of alow thermal conductivity material, a sleeve of a rough and porous heatinsulating material .mounted upon said core at an intermediate portionthereof, a pair of tubular metallic sheaths fitted over said core, saidsheaths having approximately the same diameter as said sleeve, aplurality of turns of a thermionic filament helically wound upon saidsleeve and a portion of said metallic sheaths, said filament beingsupported by and in intimate contact with said sheaths.

4. Ruggedized electrode structure for electron discharge devicescomprising a core of a low thermal conductivity material, a sleeve of arefractory material mounted on an intermediate portion of said core, apair of rigid metallic members fitted on said core and in abuttingrelation with said sleeve, and a metal filament wound helically upon,said metallic members and sleeve, said metallic members serving toreinforce said core, support said filament and conduct current thereto.

No references cited.

